In recent years, Ethernet (registered trademark) has been used in not only LANs (Local Area Networks) but also carrier networks as an infrastructure technique which supports packet networks. Therefore, Ethernet (registered trademark) has been used in data communication and has also been used in communication which is sensitive to variation of transmission delay and data loss in transmission such as transmission of audio and moving images. When the communication which is sensitive to variation of transmission delay and data loss in transmission is performed, monitoring of transmission quality is important. As an elemental technology of monitoring transmission quality, a time synchronization protocol and a PTP (Precision Time Protocol) which are prescribed in the IEEE1588 have been used. Devices which execute the PTP operate as masters or slaves. A PTP master has a master clock which supplies a clock for time synchronization to a network. A PTP slave has a slave clock which is synchronized with the master clock. The PTP slave detects a time difference between the master clock and the slave clock and attains time synchronization so as to maintain time synchronization in the network. Furthermore, the PTP slave detects a difference between an operation frequency of the master clock and an operation frequency of the slave clock so that the operation frequency of the slave clock coincides with that of the master clock. The related art is disclosed in Japanese Laid-open Patent Publication No. 2007-174680.